Urinary stone disease, urolithiasis, affects about 2 to 3% of the general population in the United States and other industrialized countries (1). In some population groups the occurrence can be significantly higher. For example, the prevalence of urolithiasis among adults in Taiwan reaches 8 to 9%, and urinary stones were reported to be the third most common disease in northern Italy (2,3). The incidence of urinary stones further increases with age, in part, due to age-related conditions such as arterial hypertension (4). The likelihood that a Caucasian male will develop stone disease by age 70 is about 1 in 8 (1). While extracorporeal shock wave lithotripsy has-simplified urinary stone removal, the recurrence rates remain high, reaching 50% to 70% in 10 years (5,6).
Since a majority of urinary stones (75-80%) are made of calcium oxalate, the control of concentrations of calcium and/or oxalate in urine is an important part of a medical treatment program to prevent stone formation or recurrence. Hypercalciuria is more common in patients with recurrent calcium oxalate renal stones; it is found in 50% of the cases, compared to about 35% for mild hyperoxaluria (1). However, the lowering of urinary oxalate level has a number of advantages. The contribution of oxalate to calcium oxalate saturation is considerably greater than that of calcium. As a result, a relatively small decrease in oxalate concentration could lower the calcium oxalate level below saturation, and thus prevent stone formation. In addition, changing the calcium concentration in urine is difficult, and risks increased oxalate absorption; it may also affect important physiological processes, such as bone calcification.
Control of urinary oxalate through the diet can produce only a partial effect, because dietary oxalate contribution to urinary oxalate is only 8 to 40% (1,7), with the rest of it synthesized endogenously, mainly in liver. The key reactions of oxalate synthesis are the conversion of glycolaldehyde to glycolate by aldehyde dehydrogenase, the conversion of glycolate to glyoxylate by glycolate oxidase and the oxidation of glyoxylate to oxalate by lactate dehydrogenase. Several minor reactions such as catabolism of hydroxyproline and degradation of aromatic amino acids may also contribute to glyoxylate and oxalate synthesis. However, in vivo, the combined contribution from these reactions is probably less than 5% (8). Glyoxylate can be converted back to glycolate by the action of D-glycerate dehydrogenase (8).
The most extreme example of a disease in which an increase in metabolically derived oxalate leads to urinary stone formation is the clinical syndrome of primary hyperoxaluria. Type I primary hyperoxaluria is characterized by a genetic defect in the peroxisomal vitamin B6-dependent enzyme alanine:glyoxylate aminotransferase (AGT). This defect results in decreased conversion of glyoxylate to glycine; consequently, conversion of glyoxylate to oxalate increases. Type II primary hyperoxaluria is caused by the deficiency of D-glycerate dehydrogenase (8). Patients with these disorders have severe renal stone disease leading to renal failure, and will eventually require renal and hepatic transplantation. Lowering urinary oxalate in these patients can alleviate the severity of stone disease and avoid costly transplantation procedures.
Acquired hyperoxaluria may be caused by a number of factors, such as the gluttony for oxalate-rich foods, abuse of ascorbic acid (vitamin C), and by small bowel disease or bypass of the small intestine that causes increased colonic oxalate absorption. In these cases, lowering urinary oxalate concentration can significantly diminish the probability of stone formation.
About 25% of all stone formers have no quantifiable abnormalities of urinary composition. This condition, called idiopathic stone disease, is likely caused by low activity of normal urinary inhibitors of crystal nucleation and crystal growth. Even though urinary oxalate in these individuals is within a normal concentration range, lowering it will decrease the contribution of oxalate to calcium oxalate supersaturation, and thus lower the probability of stone formation.
Development of an effective drug therapy that decreases urinary oxalate concentration can be a valuable tool in the prophylaxis and treatment of urinary stone disease. Several pharmacological approaches have been tested in an attempt to develop such therapy. One approach is to inhibit the enzymes involved in oxalate biosynthesis. Several inhibitors of either aldehyde dehydrogenase or glycolate oxidase were tested in both animals and humans with mixed results (9,10). Newer inhibitors of aldehyde dehydrogenase may also be used (11,12). However, one drawback of this approach may be the accumulation of glycolaldehyde, a potentially toxic agent.
Another approach to reduction of urinary oxalate concentration is based on the use of the vitamin B6 precursor pyridoxine. This treatment benefits only a small number of patients with vitamin B6-dependent type I primary hyperoxaluria (13). The mechanism of this effect is not clear, but may be related to the ability of pyridoxal-5xe2x80x2-phosphate to modulate protein expression, in particular, the expression of AGT (14). The reports on the effects of pyridoxine in individuals without Type I primary hyperoxaluria are controversial. The intake of pyridoxine in doses of 40 mg/day is associated with reduced risk of kidney stone formation in women (15) but not in men (16). On the other hand, prescription of 200 mg of pyridoxine daily did not reduce urinary oxalate levels in stone formers (17). If the primary mode of pyridoxine action is the regulation of AGT expression and/or activity, these controversial results may reflect individual differences in enzyme status and, thus, may render many stone formers non-responsive to pyridoxine treatment.
The most recent development in the field has been with an approach to trap glyoxylate in liver and reduce the amount available for conversion to oxalate. It is suggested that this approach has a significant potential because of the proximity of glyoxylate to the terminal step in oxalate synthesis. One of the proposed treatments relies on the reactivity of the glyoxylate carbonyl group with the free sulphydryl group of cysteine. The cysteine precursor, (L)-2-oxothiazolidine-4-carboxylate (OTZ) is used as the therapeutic agent because of its low toxicity. OTZ has been shown to decrease urinary oxalate concentration in a rat model of hyperoxaluria (18). Treatment with OTZ also resulted in decreased urinary oxalate levels in normal individuals (19). However, at elevated levels, free cysteine can interfere with a variety of reduction-oxidation reactions in the cell, and is potentially cytotoxic. In a human study, OTZ administered to patients for only 48 hours, produced a number of the mild to moderate adverse effects (19).
Based on all of the above, there is a need in the art for effective methods to treat and prevent urinary stone disease.
The present invention provides methods and pharmaceutical compositions for treating or inhibiting urinary stone disease that comprise administering to an individual with urinary stone disease or at risk of developing urinary stone disease an amount effective of pyridoxamine to reduce urinary oxalate concentrations. In one set of embodiments, the methods comprise treating an individual suffering from urinary oxalate stones. In another set of embodiments, the methods comprise treating an individual at risk for urinary stone disease in order to reduce or prevent formation of urinary stones in the individual.
In one aspect, the present invention provides methods for treating an individual with urinary stone disease with an amount effective of pyridoxamine to reduce urinary oxalate concentrations. As used herein, the term xe2x80x9curinary stone diseasexe2x80x9d (urolithiasis) means a condition characterized by stone formation in the urine. Conditions leading to urinary stone disease include, but are not limited to the elevated excretion of urinary oxalate (hyperoxaluria), elevated excretion of urinary calcium (hypercalciuria), low excretion of urinary citrate (hypocitraturia), and low activity of the inhibitors of calcium oxalate crystal nucleation and growth.
As used herein, the term xe2x80x9curinary oxalatexe2x80x9d means oxalic acid in an individual""s urine.
In a preferred embodiment of this aspect of the invention, the urinary stone disease comprises the production of calcium oxalate urinary stones. In a further embodiment, the individual has hyperoxaluria, an elevated urinary oxalate excretion. As used herein, the term xe2x80x9celevated urinary oxalate excretionxe2x80x9d means urinary oxalate excretion exceeding 45 mg per day per individual. Normal levels of oxalate a considered to be 8-45 mg/day, so levels that exceed 45 mg/day constitute hyperoxaluria. These levels can reach 90 to 270 mg/day in patients with primary hyperoxaluria.
In another example, the hyperoxaluria comprises primary hyperoxaluria. As used herein, the term xe2x80x9cprimary hyperoxaluriaxe2x80x9d means a genetic defect predisposing an individual to hyperoxaluria, including but not limited to Type I and Type II primary hyperoxaluria.
As used herein, xe2x80x9cType I hyperoxaluriaxe2x80x9d means a genetic defect in the peroxisomal vitamin B6-dependent enzyme alanine:glyoxylate aminotransferase (AGT). This defect results in decreased conversion of glyoxylate to glycine; consequently, conversion of glyoxylate to oxalate increases.
As used herein, xe2x80x9cType II hyperoxaluriaxe2x80x9d means a genetic deficiency of D-glycerate dehydrogenase.
In a further embodiment, the hyperoxaluria comprises acquired hyperoxaluria. As used herein xe2x80x9cacquired hyperoxaluriaxe2x80x9d means hyperoxaluria that is not due to a genetic inheritance of the disease, and includes hyperoxaluria due to the gluttony for oxalate-rich foods, abuse of ascorbic acid (vitamin C), or due to small bowel disease or bypass of the small intestine that causes increased colonic oxalate absorption.
In another embodiment, the individual has hypercalciuria, an elevated excretion of urinary calcium. As used herein, xe2x80x9celevated excretion of urinary calciumxe2x80x9d means excretion of more than 300 mg calcium/day/individual or greater than 4 mg calcium/kg body weight/day. Major causes of hypercalciuria include vitamin D-dependent increase in intestinal calcium absorption associated with primary hyperparathyroidism, i.e. excessive production of parathyroid hormone (serum hormone concentration is  greater than 65 pg/ml), and low calcium re-absorption by the kidney.
In another embodiment, the individual has hypocitraturia, low excretion of urinary citrate. Urinary citrate forms a soluble salt with calcium, thus reducing the amount of free calcium available to form calcium oxalate. As used herein, the term xe2x80x9clow excretion of urinary citratexe2x80x9d means excretion of less than 200 mg of citrate/day/individual.
In another embodiment, the individual has idiopathic stone disease caused by low activity of the inhibitors of calcium oxalate crystal nucleation and growth. Such inhibitors include, but are not limited to the glycoproteins nephrocalcin and osteopontin, which interact with crystals and inhibit their growth. Their low activity is presumably due to genetically-based abnormalities in protein structure. An average level of nephrocalcin in the urine is 15 mg/dl.
Pyridoxamine can be administered as the sole active pharmaceutical agent, or it can be used in combination with one or more other agents useful for treating urinary stone disease, including but not limited to (L)-2-oxothiazolidine-4-carboxylate (OTZ), allopurinol, inhibitors of aldehyde dehydrogenase, inhibitors of glycolate oxidase, nephrocalcin and osteopontin; or in combination with therapies such as extracorporeal shock wave lithotripsy and reduced oxalate level diets. As used herein, the term xe2x80x9creduced oxalate level dietxe2x80x9d means diet from which oxalate-rich foods (rhubarb, spinach and other leafy vegetables, cashews, almonds, and strong tea) have been eliminated.
As used herein, the term xe2x80x9cextracorporeal shock wave lithotripsyxe2x80x9d means the in situ fragmentation of stones in the kidney, renal pelvis, or ureter by exposing them to ultrasonic waves.
When administered as a combination, the therapeutic agents can be formulated as separate compositions that are given at the same time or different times, or the therapeutic agents can be given as a single composition.
In another aspect, the present invention provides methods for preventing urinary stone disease in an individual at risk of developing urinary stone disease, by administering to the individual an amount effective of pyridoxamine to reduce urinary oxalate concentrations. As used herein, an individual is xe2x80x9cat risk of developing a urinary stone diseasexe2x80x9d if one or more of the following conditions apply: they have previously had urinary stone disease; they have an elevated urinary oxalate excretion, as defined above; they have a genetic predisposition to primary hyperoxaluremia as defined above; they have elevated excretion of urinary calcium (hypercalciuria) as defined above, they have lowered excretion of urinary citrate (hypocitraturia) as defined above, or they have low levels of the inhibitors of calcium oxalate crystal nucleation and growth, as defined above.
In various preferred embodiments of this aspect of the invention, the individual is further treated with one or more compounds selected from the group consisting of (L)-2-oxothiazolidine-4-carboxylate (OTZ), allopurinol, inhibitors of aldehyde dehydrogenase, inhibitors of glycolate oxidase, nephrocalcin and osteopontin; and/or with a reduced oxalate level diet.
The pyridoxamine may be made up in a solid form (including granules, powders or suppositories) or in a liquid form (e.g., solutions, suspensions, or emulsions). Pyridoxamine may be applied in a variety of solutions and may be subjected to conventional pharmaceutical operations such as sterilization and/or may contain conventional adjuvants, such as preservatives, stabilizers, wetting agents, emulsifiers, buffers etc.
For administration, the pyridoxamine is ordinarily combined with one or more adjuvants appropriate for the indicated route of administration. Pyridoxamine may be admixed with lactose, sucrose, starch powder, cellulose esters of alkanoic acids, stearic acid, talc, magnesium stearate, magnesium oxide, sodium and calcium salts of phosphoric and sulphuric acids, acacia, gelatin, sodium alginate, polyvinylpyrrolidine, and/or polyvinyl alcohol, and tableted or encapsulated for conventional administration. Alternatively, the pyridoxamine may be dissolved in saline, water, polyethylene glycol, propylene glycol, carboxymethyl cellulose colloidal solutions, ethanol, corn oil, peanut oil, cottonseed oil, sesame oil, tragacanth gum, and/or various buffers. Other adjuvants and modes of administration are well known in the pharmaceutical art. The carrier or diluent may include time delay material, such as glyceryl monostearate or glyceryl distearate alone or with a wax, or other materials known in the art.
Pharmaceutical compositions containing pyridoxamine are administered to an individual in need thereof. In therapeutic applications, for example, pyridoxamine is administered to an individual suffering from urinary stone disease in an amount sufficient to reduce urinary oxalate concentrations, and to thereby reduce or eliminate the occurrence of calcium oxalate urinary stones. Amounts effective for this use depend on factors including, but not limited to, the route of administration, the stage and severity of the urinary stone disease, the general state of health of the individual, and the judgment of the prescribing physician. Pyridoxamine is safe and effective over a wide dosage range. However, it will be understood that the amount of pyridoxamine actually administered will be determined by a physician, in the light of the above relevant circumstances.
Pyridoxamine may be administered by any suitable route, including orally, parentally, by inhalation or rectally in dosage unit formulations containing conventional pharmaceutically acceptable carriers, adjuvants, and vehicles, including liposomes. The term parenteral as used herein includes, subcutaneous, intravenous, intraarterial, intramuscular, intrasternal, intratendinous, intraspinal, intracranial, intrathoracic, infusion techniques, intracavity, or intraperitoneally. In a preferred embodiment, pyridoxamine is administered orally.